In order to explain the background of the invention, reference will be particularly made to FIG. 1. The reference numerals 1,2, and 3 designate a thermionic cathode, a cylindrical focus electrode and an accelerating anode, respectively, and all of them are arranged so as to produce electron beams 6. The focus electrode 2 is commonly known as a "Wehnelt electrode". The reference numeral 4 designates a heater for heating the cathode 1. The reference numeral 5 designates a vacuum container.
The device of FIG. 1 is operated as follows:
The cathode 1 is heated by the heater 4, and high-voltage pulses are impressed between the anode 3 and the cathode 1, and further between the anode 3 and the focus electrode 2, thereby giving rise to an emission of electron beams 6 from the cathode 1. The electron beams 6 are produced to form a predetermined shape by the function of the focus electrode 2.
However, in operation, a high electric field may occur at the respective portions of the anode 3 and the focus electrode 2 which portions have the least distance therebetween, and in some cases it causes a discharge between confronting surfaces of the two portions. Furthermore, the surface of the focus electrode 2 becomes rough which is caused by a large current in the discharge, and then the electric field thereat increases, resulting in an increased possibility of the discharge.
On the other hand, in the midst of the operation, the cathode material always thermally-vapourizes from the cathode 1, and the vapourized material attaches to the surface of the focus electrode 2. The portion of the focus electrode 2 which is located close to the cathode 1 is in an elevated temperature due to the radiant heat from the cathode 1, and hot electrons are repeatedly emitted from the focus electrode 2 accompanied with a decrease of work function caused by the attachment of cathode material to the focus electrode 2.
Under such construction, there are disadvantages in that a discharge is likely to occur between the cathode and the anode, unfavourably affecting its essential operation, and additionally that electrons are repeatedly emitted from the focus electrode 2.